This invention relates to lasers and more particularly to low-powered dual frequency gas lasers the output of which may be rapidly switched between the two frequencies in a controlled manner.
Low-powered gaseous lasers are well known and are widely used in a number of applications where their high degree of temporal and/or spatial coherence is of advantage. For a number of these applications, there is a need to be able to operate the laser system alternately at a pair of frequencies.
For example, a common technique for assaying a sample for the presence of or measuring the concentration of a wide variety of specific substances makes use of the selective absorption of radiation of different optical frequencies by the sample. In many instances, this technique may be simplified to observing the transmission of the sample at a pair of frequencies, one of which is strongly absorbed by the specific substance of interest but is not appreciably absorbed by other substances which may be present in the sample, and the other of which is absorbed by neither the substance of interest nor the rest of the sample. Non-dispersive selective absorption meters designed to make such observations are finding increasing application in such areas as process control, pollution monitoring, and the like. Such meters may employ lasers as the source of radiation, thereby taking advantage of the laser's high spectral resolution with a consequent surer discrimination between substances having similar or complex absorption spectra. While it is possible to devise an instrument to perform the required pair of observations simultaneously, a simpler, less complex and more easily realizable instrument results if the observations are made sequentially in rapid succession. Further, while it is possible to use a pair of lasers to provide the pair of frequencies, a single laser with dual frequency output is often more desirable. Consequently, dual frequency lasers which may be made to alternate between the two frequencies in a controlled manner are of utility in non-dispersive absorption meters. An example of such an apparatus is disclosed in U.S. Pat. No. 4,059,356.
Other potential uses for time-sharing dual frequency lasers include display generation, information processing, and communications, where the two frequencies could be used, for instance, to provide separate channels or to permit frequency shift encoding.
A variety of techniques have been employed to vary the frequency of a laser. To aid in understanding the principles of these various techniques, and to better distinguish between them and the operation of the present invention, a short exposition concerning laser operation may be helpful.
Briefly, lasers are amplifiers of radiant energy which function through the emission, stimulated by photons, of like photons. (As used herein, the term "photon" means a quantum of electromagnetic radiation more generally referred to as light-i.e., ultraviolet, visible, and infrared radiation). The gain or amplification factor of such an amplifier is highly frequency dependent and depends on the physical properties of the active material of the laser. The gain is greater than unity only for those photons for which the rate of stimulated emission exceeds the rate of absorption. Such gain is only possible if the active material of the laser (1) can exist in a pair of energy states which differ from one another by the specific energy associated with the photons of interest and (2) can more densely populate the higher energy state than the lower. In the materials of interest in the practice of the present invention, the differences in energy states are those associated with transitions between energy levels by the valence electrons of atoms or ions in a gas or plasma (i.e. those transitions associated with atomic spectra).
In operation, any of a large number of means, such as d.c. or radio frequency electrical discharge in, or optical excitation of, the active gaseous material, may be used to supply the energy necessary to raise or pump electrons to the higher energy level in order to satisfy the second of these conditions.
It will be appreciated that it is possible under appropriate circumstances for both of the conditions just enumerated to be met simultaneously by more than a pair of energy levels in a single material. Further, a mix of active materials may be used to produce greater than unity gain at two or more frequencies. In such cases the gain vs. frequency curve will display a number of peaks, each centered at a frequency for which the electronic transition meets these conditions. The relative heights of the various peaks will depend upon the corresponding transition probabilities, density of the inverted populations, linewidths, etc. The profile of each individual peak will, for the cases of interest, primarily reflect the Doppler and pressure broadening of the emitted radiation (due respectively to the random thermal motion of and collisions between the individual atoms or ions of the active material).
When used as a light source, a laser is operated as an oscillator, optical feedback being provided at at least one frequency for which the total gain of the laser, including allowances for losses in the feedback loop, is greater than unity. This feedback is achieved by enclosing the laser in an optical system so arranged as to circulate photons through the laser such that their round-trip transit time is commensurate with the reciprocal of the desired frequency (i.e., the optical path length is made commensurate with the desired wavelength). Thus, in the case of the Fabry-Perot resonator, typically used for this purpose, the separation between end mirrors is chosen to be an integral number of half wavelengths, since the round-trip optical path involves two reflections. Inasmuch as any practical sized optical resonator will have a round-trip optical path many wavelengths long, the desired frequency will be a high order overtone of the fundamental frequency of the resonator, which typically will be in the gigahertz range. Thus, such a resonator at any instant is simultaneously tuned to a number of optical frequencies, each differing from the next by a single cycle per round-trip optical transit time within the resonator. To a first approximation, laser oscillation is possible only for this comb of longitudinal mode resonant frequencies, and then only for those specific frequencies which coincide with a greater than unity gain (after allowance for losses in the optical path of the resonator) of the active material.
If the mode spacing is sufficiently small compared to the width of a gain line, then oscillation may be possible at more than one of the mode frequencies. However, when oscillation occurs at a given frequency, the gain around that frequency is reduced because of the increased rate at which atoms are removed from the upper state. If the line is homogeneously broadened (i.e., if the collisional width is comparable to or larger than the Doppler width), then this effect tends to suppress oscillation of all modes except the one with highest gain; likewise, when several gain lines share a common upper level (as is the case for the He-Ne laser), oscillation will occur at the frequency having the highest gain, and oscillation at the other line will be suppressed. If the lines are not homogeneously broadened, then, in general, simultaneous oscillation at several frequencies is possible.
With this brief outline of the operation of a gas laser, it will be appreciated that switching between significantly different frequencies (i.e. frequencies separated from one another by more than the Doppler and pressure broadened linewidth) is such a laser involves operating the laser so as to alternately oscillate at a pair of frequencies corresponding to two different transitions. Two basic prior art approaches for accomplishing this switching between gain peaks may be distinguished: (1) those approaches in which the resonant cavity is fixedly tuned to simultaneously resonate such that two of its resonant frequencies correspond to a pair of greater-than-unity-gain transitions in the active material, and the optical feedback is alternately spoiled at one or the other of these frequencies; and (2) those approaches in which the resonant cavity is so dimensioned that the comb of resonant frequencies is not commensurate with the difference in frequency corresponding to such a pair of transitions, and the tuning of the cavity is then varied so as to move the comb of frequencies back and forth, thereby providing resonances alternately at one or the other frequency corresponding to the pair of transitions.
The first of the above enumerated approaches may be accomplished by varying the intracavity absorption, thereby altering the overall system gain to less than unity at the frequency absorbed, by such methods as alternately introducing and withdrawing appropriate optical filters into and out of the optical path, or changing the frequency of an interference filter (as by rocking), within the cavity. Alternatively, a movable dispersing element (such as a grating or a prism) may be placed within the cavity and oscillated or rocked back and forth so as to alternately prevent optical feedback at all but one or the other of the frequencies corresponding to greater than unity gain. In addition to requiring additional components in the optical path within the resonant cavity of the laser oscillator, such apparatus accomplishes the switching from one gain peak to the other by physically moving a component which often is highly position sensitive.
The alternative approach, that of varying the tuning of the cavity, is accomplished by altering the optical path length (i.e. the product of the geometrical length of the optical path and the index of refraction of the medium) within the resonant cavity, thereby changing the transit time and thus the comb of resonant frequencies. Such changes in path length may be accomplished by physically altering the geometrical length of the path, as by physically moving a mirror which in part defines the path, or by changing the index of refraction within at least a portion of the cavity as by altering the pressure of a gas or inducing birefringence in a crystal. These designs require either physically moving a critical component defining the cavity, the alternate pumping and evacuation of gas into and out of a portion of the cavity, or the addition of an exotic optical component to the optical train in the cavity.